Polymerization of olefins, especially ethylene, to form polyolefins is an important commercial process, literally millions of tons of such polymers being produced annually. Among the useful grades of polyethylene is a branched polyethylene formed by copolymerizing ethylene with one or more α-olefins. The resulting polyolefin is sometimes referred to as Linear Low Density Polyethylene (LLDPE), see for instance D. M. Simpson & G. A. Vaughan, “Ethylene polymers, LLDPE” Encyclopedia of Polymer Science and Technology, Vol. 2, John Wiley & Sons, New York, (online) 2005), p. 441-482, which is hereby included by reference, LLDPEs are typically made by adding to the polymerization process ethylene and one or more preformed α-olefins, and contacting them with a catalyst system that is capable of copolymerizing these olefins.
However α-olefins are more expensive than ethylene itself and so raise the cost of the resulting copolymer. Processes have been developed in which along with the copolymerization catalyst, a catalyst that can oligomerize ethylene to α-olefins is also added, thereby obviating the need for separately added α-olefins, see for instance U.S. Pat. No. 6,297,338, which is hereby included by reference. However, in many instances these oligomerization catalysts produce a series of α-olefins, e.g. 1-butene, 1-hexene, 1-octene, 1-decene, 1-dodecene, etc. The α-olefins thus produced may not be completely polymerized, and higher molecular weight olefins produced are difficult to remove from the resulting polyolefin since these higher molecular weight olefins are not very volatile. This problem has been addressed in U.S. Pat. No. 6,586,541, which is hereby included by reference. However, the process described in this patent requires at least one additional reactor space and also will produce at least small amounts of polyolefin that are different than the majority of the product. Both of these effects are undesirable. Therefore improved methods of reducing residual α-olefins in such a process are desired.
Other references that report simultaneous oligomerization and polymerization of various olefins are World Patent Application 90/15085, U.S. Pat. Nos. 5,753,785, 5,856,610, 5,686,542, 5,137,994, and 5,071,927, C. Denger, et al., Makromol. Chem. Rapid Commun., vol. 12, p. 697-701 (1991), and E. A. Benham, et al., Polymer Engineering and Science, vol. 28, p. 1469-1472 (1988).